1. Field of the Invention
The present invention relates to a variable valve timing apparatus and more particularly to a variable valve timing apparatus including an electric motor as an actuator and a control method thereof.
2. Description of the Background Art
VVT (Variable Valve Timing) has conventionally been known that changes the phase (crank angle) in (at) which an intake valve or an exhaust valve is opened/closed, according to an operating condition. Generally, a variable valve timing apparatus changes the phase by rotating, relative to a sprocket or the like, a camshaft that drives the intake valve or exhaust valve to open/close. The camshaft is rotated by such an actuator as hydraulic or electric motor.
Such a variable valve timing apparatus may be operated not only at the time of engine operation but also at the time of engine stop to change a valve timing (valve phase). As for the variable valve timing at the time of engine stop, for example, Patent Publication 1 (Japanese Patent Laying-Open No. 2004-300924) discloses control at the time of engine stop in a variable valve apparatus that continuously changes the valve timing and the valve lift amount by shifting a three-dimensional cam in the direction of a camshaft.
Specifically, the variable valve apparatus disclosed in Patent Document 1 drives valve lift amount change means and also detects the amount of shift of the cam, in a case where the camshaft need to be axially shifted at the time of engine stop. When the detected amount of shift of the cam is smaller than a set value, the driving time of the valve lift amount change means is checked. Then, if the driving time of the valve lift amount change means exceeds the set time, the driving of the valve lift amount change means is stopped thereby saving consumption power to prolong the battery life and preventing overheating resulting from overload to improve the reliability of the system.
Generally, in a vehicle on which an engine is mounted, among a variety of mounted equipment, the engine produces a relatively loud operation sound. Therefore, at the time of engine stop, the passenger is likely to perceive the operation sound of other equipment as unusual sound or noise. Therefore, the likelihood that the passenger perceives the operation sound of the variable valve timing apparatus is also preferably reduced by reducing the operation sound of an actuator such as an electric motor. However, Patent Document 1 never mentions such a problem. Although Patent Document 1 discloses that the driving time of the valve lift amount change means, namely the operation time of the actuator is restricted at the time of engine stop, the document does not make any mention of how to define the operation time.